Recently, it has been reported that a single-mode optical fiber laser or optical amplifier, which employs an optical fiber doped with a rare earth element such as neodymium (Nd), erbium (Er), praseodymium (Pr), and ytterbium (Yb), as a laser active medium (hereinafter referred to as a rare earth-doped optical fiber,) has many possible applications in wide fields such as optical sensing or optical communication, and their applicability has been expected. One example of applications thereof is an Yb-doped core optical fiber laser employing an optical fiber in which a core is doped with Yb (which is hereinafter referred to as a Yb-doped core optical fiber), which is examined for the use in marker, repairing, soldering, cutting/drilling, welding for various materials or the like, and then commercialized. Conventionally, the laser used in such material processing applications has been mainly a YAG laser, but recently the requirements for the processing performance have become more stringent, and as a result, the needs of laser performance have increased. For example,
1. a smaller spot size is required in order to achieve high precision processing;
2. a higher output power is required; and
3. a reduction in down time for maintenance, etc. of a laser (such as MTBF, and MTBM) is required.
For these requirements, the Yb-doped core optical fiber laser is characterized in that it has
1. a spot size in a μm-order;
2. a several W through several kW output power; and
3. an expected life time of 30,000 or more, and the Yb-doped core optical fiber laser has a greater advantage when compared to a conventional YAG laser.
As the rare earth-doped core optical fiber, there is generally known an optical fiber obtained by using a rare earth-doped glass, as described in Patent Documents 1 and 2. The rare earth-doped glass is doped with a rare earth element, aluminum, and fluorine in a host glass comprising a SiO2-based composition, and the rare earth-doped core optical fiber includes the glass as a core. Accordingly, the core part is doped with a rare earth element, aluminum, and fluorine.
If a SiO2 glass or a GeO2—SiO2-based glass, used for common optical fibers, is doped with about 0.1% by mass or more of a rare earth element, there occurs a problem of a so-called concentration quenching. This is a phenomenon where rare earth ions are aggregated (clustered) with each other in the glass, whereby the energy of excited electrons is likely to be lost in a non-radial process, leading to a reduction of fluorescence life time or of fluorescence efficiency. Patent Document 1 describes that by doping both of the rare earth element and Al, a high concentration of the rare earth element can be doped without causing deterioration of the light emitting characteristics, and even with a lower interaction length with the pump light, a sufficient amplification gain is attained, thereby making it possible to realize a small-sized laser or optical amplifier.
Patent Document 2 describes a method for manufacturing a rare earth-doped core optical fiber, and in particular a rare earth-doped glass. In this method, a preform of a silica porous glass having an open pore connected therewith is immersed in a solution containing a rare earth ion and an aluminum ion, and the rare earth element and the aluminum are impregnated in the preform. Thereafter, a drying process is carried out, in which the preform is dried, the chloride of the rare earth element and the aluminum are deposited in the pores of the preform, and the deposited chloride is oxidized and stabilized. Then, the preform after the drying process is sintered for vitrification. Further, at a time between the completion of the drying process and the sintering process, the preform is subject to heat treatment under an atmosphere containing fluorine to dope the fluorine.
A rare earth-doped core optical fiber is obtained by synthesizing glass, as a clad portion, around the obtained rare earth-doped glass to obtain a glass preform for manufacturing of an optical fiber; and then fiber-drawing the preform. Herein, in order to obtain an optical fiber that is used for an Yb-doped core optical fiber laser, ytterbium (Yb) may be used as a rare earth element in the manufacturing process for the rare earth-doped glass.
An example of other methods for manufacturing an Yb-doped core optical fiber is a combination of a MCVD process and a solution process, as described in Non-Patent Document 1. In this method, SiCl4, GeCl4, O2 gases, etc. are firstly flowed through a silica glass tube which is to be served as a clad glass, and a heat source such as an oxyhydrogen burner disposed outside the silica glass tube is used to oxidize SiCl4 and GeCl4 and to produce SiO2 and GeO2 glass soots, which are then deposited inside the silica glass tube. At this time, the temperature during deposition is kept to not give a completely transparent glass, thus obtaining a glass in a porous state. Next, a solution containing Yb ions is introduced into the inside of the silica glass tube having the prepared porous glass layer therein, and penetrated into the porous portion. After the sufficient penetration time with the solution, the solution is withdrawn from the silica glass tube, and the tube is dehydrated to remove water under a chlorine atmosphere. Then, the porous portion is made transparent, and core solidification is performed to prepare a preform for a Yb-doped core optical fiber. If necessary, the Yb-doped core optical fiber is obtained by synthesizing a glass, as a clad portion, around the prepared preform, thereby giving a transparent glass preform for preparation of an optical fiber; and then fiber-drawing the preform. Further, the obtained optical fiber can be used to constitute an Yb-doped core optical fiber laser.
FIG. 1 is a configuration diagram showing one example of the Yb-doped core optical fiber laser, in which the Yb-doped core optical fiber laser has a constitution comprising a Yb-doped core optical fiber 1, LD 2 as a pump light source connected to input the pump light from one end of the fiber, and optical fiber gratings 3 and 4 connected to both ends of the Yb-doped core optical fiber 1.    [Patent Document 1] Japanese Unexamined patent Application, First Publication No. 11-314935    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 3-265537    [Non-Patent Document 1] Edited by Shoichi SUDO, Erbium-doped optical fiber amplifier, The Optronics Co., Ltd.    [Non-Patent Document 2] Laser Focus World Japan 2005. 8, p.p. 51-53, published by Co., Ltd. E-express    [Non-Patent Document 3] Z. Burshtein, et. al., “Impurity Local Phonon Nonradiative Quenching of Yb3+ Fluorescence in Ytterbium-Doped Silicate Glasses”, IEEE Journal of Quantum Electronics, vol. 36, No. 8, Exit 2000, pp. 1000-1007